1. Field of Invention
The present invention relates generally to a semiconductor device and more particularly to a semiconductor device having low ON resistance.
2. Description of Related Art
Semiconductor devices having low ON resistance are used as power switching elements. For example, Japanese Patent Application No. HEI 8-213613 discloses a semiconductor device provided with n+-type drain region, n drift region, n+-type source region, and source electrode in that sequence on a drain electrode. The device is further provided with a trench gate region covered with insulating film so as to surround the n drift region from both sides.
In such construction, when negative bias voltage is applied to a gate electrode, the entire n drift region interposed between the trench gate region becomes depleted, and a potential barrier against electrons is formed on the entire n+-type source region resulting in current cutoff. On the other hand, when the negative potential of the gate electrode is decreased and potential voltage is applied, the potential barrier becomes small. As a result, electrons move from the n+-type source region to the n+-type drain region, and thus current flows. In such semiconductor devices, the channel in the ON-state is formed at the entire portion sandwiched between the trench gate rather than as an inversion layer of the gate oxide film interface, so that the mobility of the carrier is increased.
It is possible to increase the mobility of the carrier by forming the entire current path with n-type, without containing pn junction in the current path. However, in such case, the semiconductor is of unipolar type wherein only the electrons function as carriers. Therefore, the carrier density is insufficient and it is difficult to further reduce the ON resistance.
Schottky type diodes are known as devices having low ON voltage (ON resistance). However, Schottky type diodes are not used for various devices since the voltage resistance of the diode is not enough.